Giant Monitor Lizard

Giant Monitor Lizard—The giant monitor lizard was an enormous predatory reptile that prowled ancient Australia up until around 40,000 years ago. (Renata Cunha)

Scientific name: Megalania prisca Scientific classification:

Phylum: Chordata Class: Reptilia Order: Squamata Family: Varanidae

When did it become extinct? This lizard is thought to have become extinct around 40,000 years ago.

Where did it live? The remains of this animal have only been found in Australia.

Thousands of years ago, Australia was home to more than just giant marsupials. Between 1.6 million and 40,000 years ago, a giant lizard also stalked this fascinating place. Remains of the giant monitor lizard are rare, but enough remnants have been found to allow the entire skeleton of this animal to be reconstructed, and it seems that this was a true giant. The largest living lizard is the Komodo dragon (Varanus komodoensis), and in the wild, they can grow to around 3.1 m in length and 166 kg in weight—imagine a bulkier version of the Komodo dragon, which could have been anywhere up to 7 m long and more than 1,000 kg in weight. This is the giant monitor lizard, and its great size alone must have been more than enough to strike fear into the hearts of the first human inhabitants of Australia.

All living monitor lizards are carnivorous animals, and there is no reason to think that the giant monitor was any different. Sections of the animals jaw have been found, and these prove that this reptile was a meat eater as they are studded with numerous curved teeth. If the size estimates of the giant monitor lizard are true, a fully grown specimen was Australia's largest land-dwelling predator by quite some margin, and there must have been few, if any, animals that it was not capable of tackling.

Perhaps the best way of reconstructing the behavior of the giant monitor is by using the Komodo dragon as a model. This famous monitor lizard has been closely studied for years, and we know a great deal about its general biology. Like the Komodo dragon, the giant monitor probably relied on ambush to catch its prey. It may have skulked in the undergrowth near a watering hole and waited for a hapless victim to come within distance. All lizards, particularly the large monitor lizards, are incapable of maintaining a burst of speed for any significant distance. Their bodies are badly designed for long-distance running as their legs are splayed out to the side and their spines flex from side to side, which makes breathing impossible during energetic movement. In contrast, the legs of a four-legged mammal are directly beneath it, and its spine flexes up and down, which actually helps with breathing (think of a greyhound or cheetah running at full speed). So the giant monitor was limited to lightning strikes from cover, which is still a very effective technique. When the victim was a large marsupial, the giant monitor probably lunged for the neck or the soft underside, which is what the Komodo dragon does when attacking a goat or a water buffalo.

During a predatory attack, the Komodo dragon delivers a bite with its mouthful of teeth and makes no effort to cling on to its terrified prey. This is because the lizard has potent weapons: venom and saliva swarming with bacteria. A bite from a Komodo dragon usually causes a fatal infection, and the victim dies after a few days. With its powerful sense of smell, the lizard follows the scent of death to the final resting place of its prey. Recent research suggests that many types of monitor lizard are slightly venomous, and the giant monitor lizard may have been no different. In actual fact, Australia is home to a bewildering diversity of very venomous animals, and perhaps the giant monitor's saliva was poisonous as well as teeming with dangerous bacteria. We have no way of knowing for sure if this is how this lizard dispatched its prey, but the image of a 7-m-long lizard tasting the air with its big forked tongue, searching for the scent of the doomed animal it has just bitten, is a tantalizing one.

As the giant monitor lizard was so large, it could probably survive on very little food, perhaps only needing to feed once every month, or even less. However, when hunger started to bite and an attack ended in a kill, the giant monitor could have eaten a huge amount of food in one go. In a single meal, the Komodo dragon can gorge 80 percent of its own body weight in food, which is made possible by its very stretchy stomach. The Komodo dragon is also very indiscriminate when it's tearing at the dead body of its victim and everything is eaten. All the indigestible parts, that is, hair, teeth, horns, and so on, are regurgitated after digestion as a pellet smothered in foul-smelling mucus. It is possible that the giant monitor also regurgitated a stinking pellet, but on a much larger scale.

The giant monitor lizard was probably the top Australian predator, but the position of top predator in a food chain is a very precarious one as any big changes in the environment will be felt most powerfully in these lofty reaches. Something did happen around 40,000 years ago that toppled the giant monitor and many other unique Australian species. Due to global cooling, the climate of Australia is thought to have become much drier, and as the rainfall patterns changed, the vegetation began to adapt to the new climatic regimes, and much of Australia became the arid landscape we know today. As the vegetation changed, the populations of the large marsupial herbivores started to dwindle and vanish, until the giant monitor lizard had nothing left to eat. Humans may have encountered this giant lizard, and it must have been a source of wonder and fear. There is long-standing theory that humans changed the face of Australia by starting wildfires. If this occurred, the large-scale burning not only deprived the lizard's prey of food, but may have also killed the reptiles themselves and destroyed their nests.

♦ The giant monitor lizard is a favorite of cryptozoologists who believe that this reptile still haunts the Australian outback. There have been numerous sightings that people attribute to this lizard, some of which have been reported by very credible witnesses. It is worth remembering that Australia is a huge, sparsely populated place. A startling example of just what secrets this place still holds is the Wollemi pine (Wollemia nobilis), which was discovered in 1994. This living fossil had clung to existence in some remote canyons in the Blue Mountains. If a static species, such as a tree, can remain undetected during two centuries of scientific endeavor, then what are the chances of a highly mobile, albeit giant lizard, still being at large in the Australian wilderness?

♦ The larger monitor lizards spend almost all of their time on the ground, but they are proficient climbers and excellent swimmers. However, when young they prefer to spend their time in the trees as they are a tasty morsel for lots of predators, including adults of their own species. Young giant monitor lizards may have spent their early youth in the trees, well out of the way of their enormous relatives.

Further Reading: Molnar, R. Dragons in the Dust: The Paleobiology of the Giant Monitor Lizard Megalania. Bloomington: Indiana University Press, 2004; Wroe, S. "A Review of Terrestrial Mammalian and Reptilian Carnivore Ecology in Australian Fossil Faunas, and Factors Influencing Their Diversity: The Myth of Reptilian Domination and Its Broader Ramifications." Australian Journal of Zoology 50 (2002): 1-24.